The future demands for high-end CT and CV imaging regarding the X-ray source are higher power/tube current, smaller focal spots (FS) combined with the ability of active FS size, ratio and position control, shorter times for cooling down, and concerning CT shorter gantry rotation times. In addition to this, the tube design is limited in length and weight to achieve an easy handling for CV application and a realizable gantry setup for CT applications.
One key to reach higher power and faster cooling is given by using a sophisticated heat management concept inside the X-ray tube. In bipolar X-ray tubes about 40% of the thermal load of the target is due to electrons backscattered from the target, which are reaccelerated towards the target and hitting it again outside the focal spot. Hence, these electrons contribute to the temperature increase of the target and causes off-focal radiation.
Therefore one key component of the currently developed new X-ray tube generation is a scattered electron collector (SEC) located in front of the target. Such a X-ray tube comprises a source for emitting electrons, a carrier which is rotatable about an axis of rotation and which is provided with a material which generates X-rays as a result of the incidence of electrons, a heat absorbing member arranged between the source and the carrier, and a cooling system which is in thermal connection with the heat absorbing member.
The source, the carrier, and the heat absorbing member are accommodated in a vacuum space of the device. The carrier is disc-shaped and is rotatably journalled by means of a bearing. During operation, an electron beam generated by the source passes through a central cavity provided in the heat absorbing member and impinges upon the X-ray generating material of the carrier in an impingement position near the circumference of the carrier. As a result, X-rays are generated in said impingement position, which emanate through an X-ray exit window provided in a housing enclosing the vacuum space. The heat absorbing member has the same electrical potential as the carrier and is arranged between the source and the carrier to catch electrons, which are scattered back from the carrier, and to absorb radiant heat generated by the carrier when heated during operation, as a result of which the heat absorbing member is heated during operation.
To lead the heat away from the heat absorbing member, a cooling system is attached to said member, which cooling system comprises a channel for a cooling liquid, which cooling system is provided in a circumferential portion of the heat absorbing member in direct thermal contact with the heat absorbing member. The heat absorbing member is made, for example, from Mo and has a relatively large mass and volume, so that the heat absorbing member has a large heat absorbing capacity. Thus, when the device is temporarily in operation to generate X-rays of a relatively high energy level, a relatively large rate of heat absorption by the heat absorbing member temporarily occurs. Further, the rate of heat transfer from the heat absorbing member to the cooling system is limited, and the heat absorbed by the heat absorbing member is gradually transferred to the cooling system during the time that the device generates X-rays and afterwards when the device is not in operation. As a result of said gradual transfer of the heat from the heat absorbing member to the cooling system, thermal peak loads on the cooling system are prevented, so that cooling system problems, such as boiling of the cooling liquid or melting of thin-walled structures of the cooling system, are prevented.
However, the thermal load of the target is in this case determined only by electrons contributing to the tube's X-ray output. The backscattered electrons release their energy at the SEC which is integrated into the tube's cooling system. The cooling walls of the SEC are located on the outer areas at bigger radius while the heat is generated on the inner areas at smaller radius. Therefore, the inner surface of the SEC heats up and expands during an X-ray pulse while the outer part does not expand. Hence, compression stress occurs during the pulse due to the closed inner surface. While cooling down the inner surface shrinks and the stress relaxes.
In addition to the heat management contribution the SEC may act essentially as an X-ray shielding in case it is made from metal with high melting point like Mo of W.
During a high energy pulse the compression stress may increase to a value where plastic deformation results. This effect relaxes the stress during the pulse. But when cooling down the surface shrinks which causes tensile stress within the inner surface. This could result immediately in crack formation or after a series of pulses in fatigue cracks. Gas eruptions may be the result which leads to high voltage instability (arcing) and gas ionization with following ion bombardment onto the emitter (emitter failure), i.e. the target. Besides that also small particles could be separated which leads to the same results when entering the electron beam.